Allow editing of Helium values in cylinder info

Henrik Aronsen points out that we've not made it possible to edit the He
percentages for trimix diving.  It's easy enough to do, I just didn't
have any dives that needed it myself.  So here goes.

Reported-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>